Down hill jake braking, what speed and gear do you use?

Try Red Mountain Pass CO or Spruce Pine NC , great places to practice.

 

The actual speed depends upon the truck. The old adage was 1 gear lower than your ascent, but with higher power levels these days, 2 gears lower is more appropriate.

I said this many times to my students over the years.

The first truck I ever drove was an M931A2 in the Army in the 90s. 8.3C mated to a 5-speed Allison, no jakes. Coming down some of the mountains in Korea were interesting without anything extra to hold you back... so the fan switch trick helped a lot. I applied that knowledge when I was operating a 30-ton wrecker 20 years later that had an M11, 10-speed and no Jakes. Couldn't break 50 mph on the flats when pulling heavy, and the hills meant 25 mph with the fan switch on in order to properly descend. The first truck I was issues with Jakes had the NTC400 Big Cam II and a CAT 7155 16-speed transmission with pneumatic shifter. The 3-speed jakes in those trucks would put you through the windshield when empty, and would hold the truck back at 78,000 going 55 on a 6% grade. The modern DD15/DT12 combinations make it effortless, just set the cruise at whatever speed, and it will somehow hold it back.

 

16/17th with Jake on highest, depends on topography tho

 

Well , the title is downhill Jake braking ,

I’ve never used a Jake brake going uphill …..

 

Before the RTLO's came out (they shift much quicker than the older trannies) it was fairly common to use the Jake to enable quicker uphill upshifts. With the narrow powerbands back then, it wasn't uncommon to find yourself stuck in a gear and unable to upshift because you lost too much speed by the time you completed the shift and the motor couldn't pull that gear from the lower-than-hoped-for-rpm.

 

Same in the 70/80's with the old dueces & 5ton, I don't recall the their powertrains, but there were no Jake's - only tranny, transfer, & axle shiftin'.

German mountains was my interesting terrain.

I reckon I was dern good at it, as I was 1 of only 3 in the Btln that were allowed to takes troops and heavy explosives in the same load.

Yippie kaiya!

 

The old 1 gear lower adage is good and everything, but there are some places that can get you in trouble. There are a few places where you can climb up in 9th or 10th, but the backside is way steeper. Like Cabbage, or Fancy Gap, Grapevine and Tehachapi are similar. If you relied on the gear you used to crest the summit, then dropped 1 gear on any of those, and didn't catch it quick enough, you might be in for a ride.

 

I drove the M1070/M1000 HETs in Germany. 232,000 gross crawling up the grades at 6 mph and then descending the other side, timing my acceleration down the mountains to reach the top allowable speed for the weight class of 44 mph at the bottom with the 8V92TA's jakes on high singing the song of my people all the way down.

 

omg coffee everywhere! !

That's a good one

 

Yeah
I used to use the manual fan clutch switch to grab a few gears in a hurry .